This is wrong. ANY child of ANY age, flying internationally, MUST have a Nexus card or passport, straight from the US government website: "All Canadian Citizens entering the United States by AIR are required to present a valid passport, or a NEXUS card when departing from a designated Canadian airport."
So just because a couple people have gotten lucky and gotten kids across by air without a passport, does NOT mean you should plan on it. Trying to get a child across without proper identification is irresponsible.